Electromagnetic apparatus and method for nerve localization during spinal surgery

ABSTRACT

An electromagnetic pedicle awl utilizes a tightly focused time-varying magnetic flux to create a localized electromotive force (EMF) near the tip of a pedicle awl. The localized EMF creates localized eddy currents in nearby nerves which in turn excite ionic nerve channels, the excitation being detected by an electromyographic recording device. In comparison to the electrically stimulated pedicle awl of the prior art, the electromagnetic pedicle awl only excites nerves directly in front of and directly to the side of the tip. The electromagnetic pedicle awl is comprised of a tapered awl or drill tip in combination with a solid core surrounded by a solenoid. A pulsed electric current source drives the solenoid to create a time-varying magnetic field in the vicinity of the tapered tip. The awl or drill tip may be stationary with respect to the solenoid or it may rotate. The electromagnetic pedicle awl in combination with an EMG detector connected to a patient is very sensitive to the pedicle hole position with respect to adjacent nerves and reduces false placement failure.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application Ser.No. 60/849,858 entitled “Electromagnetic Pedicle Screw PlacementApparatus and Method” which was filed on Oct. 6, 2006.

COPYRIGHT NOTICE

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure as it appears in the Patent and TrademarkOffice patent file or records, but otherwise reserves all copyrightrights whatsoever.

FIELD OF INVENTION

The present invention relates generally to the avoidance of nervoussystem injury during surgery for correction of spinal column injuries,degeneration and deformities in the fields of neurosurgery andorthopedics. More specifically, the invention is used for the placementof medical instrumentation apparatus into and between spinal vertebrae.

BACKGROUND OF THE INVENTION

Spinal conditions such as degenerative disc disease or spondylolisthesiscan cause signs and symptoms that include back or lower extremity pain,muscle spasms, weakness, dysfunction of bowel and/or bladder, and gaitdisturbance.

To correct these and other similar conditions of vertebral dislocation,the only effective long-term curative treatment may be achieved byfusion of the affected vertebra to its adjacent neighbor. Vertebralfusion is generally augmented by instrumentation, or fixing apparatus,to and between vertebrae. Transpedicular fixation is a particularlyimportant process in the treatment of spinal conditions that requirevertebral fusion. In addition to the stabilization and correction ofspondylolisthesis, other spinal conditions may be treated bytranspedicular fixation: stabilization of fractures, correction ofspinal deformities (scoliosis, kyphosis), stabilization and correctionof degenerative spinal lesions, reconstruction after tumor resection,and secondary spinal surgery.

In FIG. 1, a drawing of the human spine shows that the spinal column 1is comprised of a number of vertebrae, categorized into four sections ortypes: the lumbar vertebrae 2, the thoracic vertebrae 3, the cervicalvertebrae 4 and the sacral vertebrae 5. Starting at the top of thespinal column 1, the cervical vertebrae 4 are labeled 1^(st) cervicalvertebra (C1) through 7^(th) cervical vertebra (C7). Just below the7^(th) cervical vertebra is the first of twelve thoracic vertebrae 3labeled 1^(st) thoracic vertebra (T1) through 12^(th) thoracic vertebra(T12). Just below the 12^(th) thoracic vertebrae 3, are five lumbarvertebrae 2 labeled 1^(st) lumbar vertebra (L1) through 5^(th) lumbarvertebra (L5), the 5^(th) lumbar vertebra being attached to the sacralvertebrae 5 (S1 to S5), the sacral vertebrae 5 being naturally fusedtogether in the adult.

Spinal fusion surgery typically involves the corrective fusion of lumbarvertebrae 2, of which a representative transverse drawing of such avertebra is shown in FIG. 2. Representative lumbar vertebra 10 has anumber of notable features which are in general shared with the thoracicvertebrae 3 and cervical vertebrae 4, although the feature thicknessesand shapes may alter between the various types of vertebrae. The thickoval segment of bone forming the anterior aspect of the vertebra 10 isthe vertebral body 12. Vertebral body 12 is attached to a bony vertebralarch 13 through which the neural elements run. Vertebral arch 13,forming the posterior of vertebra 10, is comprised of two pedicles 14,which are short stout processes that extend from the sides of vertebralbody 12, and two laminae 15, the broad flat plates that project frompedicles 14 and join in a triangle to form a hollow archway, thevertebral foramen 16 or spinal canal. The spinous process 17 protrudesfrom the junction of laminae 15: these are the ridges that can be feltthrough the skin along the back of the spine. Transverse processes 18project from the junction of pedicles 14 and laminae 15. The structuresof the vertebral arch protect the spinal cord and/or spinal nerves thatrun through the spinal canal.

The pedicles of typical lumbar vertebra increase in sagittal width from9 mm to up to 18 mm at L5 (lowest lumbar vertebra). They increase inangulation in the axial plane from 10 degrees at L1 to 30 degrees by L5.Pedicles of the thoracic and cervical vertebra are typically smaller—assmall as 4 mm in an adult. Pedicle widths in adolescents areproportionally smaller. The pedicles exhibit good mechanical strength incomparison to the other vertebral features.

Pedicles are used as a portal of entrance into the vertebral body forfixation with pedicle screws for placement of stabilizing rods or platesfor fusion as shown in FIGS. 3 a and 3 b. Pedicles are used due to theirstrength, size and proximity to ease of entrance through the posteriorside of the human body. In FIG. 3 a, a transverse view of vertebra 10 isshown including pedicular holes 25 are drilled and tapped with pedicularscrews 20 inserted into the vertebral body 12 through pedicles 14.

Typical pedicle screws are made of titanium alloy, are MRI compatible,and are highly resistant to corrosion and fatigue. Pedicle screws havelength ranges from 30 mm to 60 mm. The threaded (major) diameter rangesfrom 4.5 mm to 8.5 mm.

The transpedicular fixation process is accomplished by placing pediclescrews into the pedicular region of adjacent vertebrae, as in FIG. 3 a,and attaching rods between the pedicle screws to stabilize the vertebraewith respect to each other. In FIG. 3 b, a second vertebral structure,in this case a sacral vertebrae S1 is shown to be connected to L5vertebra through a set of pedicle screws 23 inserted into L5, a set ofpedicle screws 21 inserted into S1; the pedicle screws 23 and 21 beinginterconnected by a pair of rods 22. Rods 22 may be further connected toeach other by a transverse rod 24. The transpedicular fixation is formedby pedicle screws 23, pedicle screws 21, rods 22 and transverse rod 24which fixes the vertebra L5 and S1 with respect to each other, therebyallowing the vertebrae to fuse in the healing process. If not placedproperly, the pedicle screws 23 (or pedicle screws 21) may breach thevertebral walls thereby leading to nerve root damage or pressure on thespinal cord. The mechanical integrity of the screw placement must alsobe sufficient to support the rod structures and remain intact undermechanical stress. The present invention is an apparatus and method thatsignificantly improves the placement of pedicle holes (and screws) sothat vertebral walls are not breached and also promotes mechanicalintegrity and avoidance of neurological injury.

The method of placement of pedicle screws and fixtures in the prior artis typically done under open surgical operation where the patient'sspine is exposed. More recently, percutaneous placement of pediclescrews and fixtures has become more commonplace, causing less tissuedamage and allowing for more rapid healing. During each prior artmethod, a pilot hole, using a pedicle awl or drill, is carefully madethrough each pedicle and into the vertebral body. The hole is threadedand a pedicle screw inserted.

The position and angle of the pilot hole is crucial to a successfulpedicle screw placement and can be difficult to achieve, especially inpercutaneous operations. If the pilot hole breaches the pedicle wall,spinal nerve damage may occur accompanied by chronic pain in thepatient. Worse yet, if the pedicle wall is breached medially in thecervical or thoracic spine, permanent spinal cord damage may result.Also, a breach will typically weaken the mechanical integrity of thepedicle screw fixture.

In the prior art, the ability to accurately judge the integrity of thepedicle is generally limited to post-operative observation. For example,pedicle screw misplacements are usually detected only when pain orneurological deficit is reported by the patent. Corrective surgery toreposition the malpositioned pedicle screw is expensive, carriesinherent medical risks, and may not reverse the neurological deficit orpain.

Radiographic imaging and computer-assisted methods have been developedin the prior art to increase the probability of success in the operativeprocess. For example, fluoroscopic imaging, essential to percutaneouspedicle screw placement, is used to take lateral and anterior-posteriorimages of the vertebrae and to guide wire placement. After guide wireshave been inserted through the pedicle and into the vertebral body, apilot hole is made using a cannulated pedicle awl. But placement errorsstill occur. While flouroscopic-assisted pedicle screw guidancedecreases the risk of misplacement such equipment is expensive and isnot universally available. Hence the methods developed in the prior arthave not been entirely successful.

Referring to FIGS. 4 a and 4 b, it is known in the prior art tostimulate and detect electromyographic (EMG) signals generated fromnerve roots that course along the outer surface of the pedicles. In sucha prior art method an electrostatic pedicle awl 100 is charged with acurrent source 105. The current supplied from the current source createsan electric field 125 near the tip of the electrostatic awl which inturn causes charge migrations in nerve ion channels in the vicinity ofthe tip area shown as the region of nerve excitation 130.

In use, the electrostatic pedicle awl 100 of the prior art is initiallypositioned by sight or by fluoroscopic imaging onto a pedicle 120 ofvertebra 115 and then rotated using a removable handle 110 and lightlytapped with a hammer (not shown) to create the pilot hole. An EMG 150 istaken of muscles that respond to specific nerves in the vicinity of thepedicle wall 122. As electrostatic pedicle awl 100 moves into pedicle120, EMG 150 is monitored for nerve excitations. If the tip ofelectrostatic pedicle awl 100 or its associated electric field 125breaches the pedicle wall 122, nerve channels are excited and the EMGreacts, generating signals that alert the surgeon to the breach. Inpractice, the surgeon then withdraws electrostatic pedicle awl 100 andeither repositions it or abandons the site altogether.

U.S. Pat. No. 6,796,985 to Bolger, et al. essentially describes anelectrostatic pedicle awl with special emphasis on the detection ofsignals from muscles during the drilling process and operational aspectsof generating alerts.

One shortcoming of the prior art electrostatic pedicle awl is that theelectric field generated is not well contained. The lack of containmentof the electric field often results in false readings. For example, incases where the pedicle has been previously breached and the awl hasbeen redirected, EMG signals may persist despite a corrected trajectory.Inaccurate EMG signals often lead to false placement failure andunnecessary alternative fixation procedures. Unnecessary procedurescause higher patient morbidity and potential liability.

These similar problems exist in other prior art surgical procedures inwhich avoidance of nerves is critical, including the extreme lateralinterbody fusion procedure (“XLIF procedure”). The XLIF procedure is amethod of achieving a direct lateral approach to the intervertebral discspace through the psoas muscle. To perform the XLIF procedure, it isnecessary to insert a retractor through the muscle, while avoiding thenerves, to provide an operative corridor to the spine and intervertebraldisc space. Once access is achieved, pathology may be addressed as wellas insertion of interbody implants.

The psoas muscles, crucial for hip flexion, are found in the lumbarregion and are anchored on either side of the spine. The muscles extendinto the pelvic area and attach to the hip. Critical nerve roots coursewithin the psoas muscle and must be avoided during surgery to avoiddamage and resulting pain or paralysis.

SUMMARY OF INVENTION

One preferred embodiment of the invention includes apparatus and methodfor creating holes in vertebral pedicles in preparation fortranspedicular fixation and avoiding nerves during surgical procedures.The apparatus disclosed includes an electromagnetic awl tool capable ofproducing time varying magnetic fields. Among other components, theelectromagnetic awl is comprised of a core, a tapered awl tip attachedto the core and a solenoid assembly for producing a magnetic field. Anautomatically varying electric current source is provided whichgenerates a time varying magnetic field in the coil. A mechanism is alsoprovided for detecting excitation of nerve cells by an electric fieldinduced by the time varying magnetic field.

In one preferred embodiment, the awl tip is fixed with respect to thesolenoid. In this embodiment, a gripping handle is provided attached tothe solid core. The current in the solenoid generates a magnetic field.The magnitude and time-derivative of the current at the current sourceare adjustable so that the magnitude and time derivative of magneticflux generated is correspondingly adjustable. The current source canproduce a set of critically damped or over-damped current pulses. Abipolar alternating current may be produced in another embodiment.

An electromyograh (EMG) is utilized for detecting the excitation ofnerve cells due to the breaching or near breaching of a pedicle wall inthe vertebral pedicle. A number of preferred locations for the EMGelectrodes are taught wherein the nerve excitation signals are bestdetected; the locations being in the lower extremity of the body andparticularly in the vastus lateralis muscle, the medial gastrocnemiusmuscle and the tibialis anterior muscle.

In an alternate embodiment, an electromagnetic awl is comprised of acore bushing, a tapered metal drill tip inserted into the solid corebushing, a solenoid assembly for producing a magnetic field furthercomprised of a wire looped around the core bushing, an electric currentsource attached to the wire so that a time varying magnetic field isgenerated in the vicinity of the metal drill tip, and a mechanism fordetecting the excitation of nerve cells by the time varying magneticfield.

In an alternate embodiment, the drill tip is made to rotate with respectto the solenoid; the rotation being accomplished manually by gripping ahandle mechanically attached to the drill tip or by a motor or robot endeffector attached to the drill tip.

In an alternate embodiment, the electromagnetic awl is comprised ofmultiple ceramic core solenoids aligned with parallel axes and who sharea common metal drill tip. Each of the separate solenoids produces amagnetic field independently or in conjunction with the other solenoidsproducing combined electromagnetic effects. Addressing the solenoids ina time varying manner produces a magnetic field with a directionalcapability with respect to the tapered metal drill tip.

In another alternate embodiment, multiple ferrous rods having solenoidsin the handle are mounted with parallel axes and share a single drilltip. By indexing solenoids in the handle, directional magnetic fieldscan be produced at the drill tip.

In yet another embodiment, the electromagnetic awl is comprised of asingle internal solenoid. In this embodiment, the winding of thesolenoid is accomplished in a manner which directionally biases themagnetic field that emanates from the drill tip.

In another alternate embodiment, the electromagnetic awl is providedwith a ferrous rod with a solenoid assembly in the handle for producinga magnetic field at the tip of the rod.

Yet another embodiment includes a probe or stylus for passing throughsoft tissues without an associated drill tip.

In yet another embodiment, the probe includes a series of removable tipsand cores to accommodate use of differing tools and cannulatedimplements.

The current generated in the solenoid from the current source in thealternate embodiment generates a magnetic field, the magnitude andtime-derivative of the current at the current source is manuallyadjustable so that the magnitude and time derivative of magnetic fluxgenerated is correspondingly adjustable. The current source produces aset of critically damped or over-damped current pulses or in analternate embodiment, a bipolar alternating current

In one embodiment, an electromyograph (EMG) is utilized for detectingthe excitation of nerve cells due to the breaching or near breaching ofa pedicle wall in the vertebral pedicle. A number of preferred locationsfor the EMG electrodes are taught wherein the nerve excitation signalsare best detected; the locations being in the lower extremity of thebody and particularly on the vastus lateralis muscle, the medialgastrocnemius muscle and the tibialis anterior muscle.

One preferred method disclosed includes use of an electromagnetic awl tocreate a hole in a vertebral pedicle of a vertebra comprised of thesteps of connecting an electromyograph (EMG) to the human body withelectrodes placed in the muscles of the lower extremities, placing anelectromagnetic awl against a vertebral pedicle, activating theelectromagnetic awl to produce an electromagnetic field, rotating thedrill tip of the electromagnetic awl to cut and remove material from thegiven vertebral pedicle, monitoring the EMG for nerve excitation, movingthe drill tip within the vertebral body, ceasing movement if an EMGexcitation in the muscles of the lower extremities is observed, andremoving the electromagnetic awl tool from the vertebral pedicle.

Another preferred method disclosed includes forming a hole is a psoasmuscle utilizing an electromagnetic awl including the steps ofconnecting an EMG to muscles of the lower extremity, monitoring the EMGfor nerve excitation, bringing the electromagnetic awl in contact withthe psoas muscle, activating the electromagnetic awl to produce anelectromagnetic field, advancing the awl in a drilling process to createa passage through a psoas muscle, aborting the drilling process if anEMG excitation is observed, redirecting the awl tool to continuedrilling, and removing the awl tool from the psoas muscle.

In the preferred method the drill tip is rotated manually. In analternative preferred method the drill tip is rotated automatically by amotor of electric or pneumatic means.

Another preferred embodiment includes providing an electromagnetic awlwith multiple cores and/or specially wound coils and/or Faraday shieldsto directionally orient the electromagnetic field with respect to theaxis of the electromagnetic awl. In a still further embodiment, anelectronic controller sweeps the intensity and position of theelectromagnetic field with respect to the axis of the electromagneticawl while simultaneously displaying the position of the sweep in orderto aid in location of the pedicle wall and/or other sensitive nervetissues.

BRIEF DESCRIPTION OF DRAWINGS

The disclosed inventions will be described with reference to theaccompanying drawings, which show important sample embodiments and whichare incorporated in the specification hereof by reference, wherein:

FIG. 1 is a pictorial drawing of the human spine showing the varioustypes of vertebrae;

FIG. 2 is transverse drawing of a lumbar vertebra indicating themorphology of vertebrae significant to the present invention;

FIG. 3 a is a transverse drawing of a general vertebra indicating theplacement of pedicle screws in relation to the present invention;

FIG. 3 b is an isometric view of lumbar vertebra L5 attached to sacralvertebra S1 via a transpedicular fixation device in relation to thepresent invention;

FIG. 4 a is schematic drawing of a prior art device, namely a pulsedelectrostatic pedicle awl in contact with a vertebral pedicle;

FIG. 4 b is a schematic drawing of an electromyographic (EMG) instrumentused in conjunction with a pulsed current pedicle awl;

FIG. 5 is a schematic drawing of an electromagnetic pedicle awl incontact with a vertebral pedicle in the preferred embodiment of thepresent invention;

FIG. 6 is a cross-section view of a manual electromagnetic pedicle awlin the preferred embodiment of the present invention;

FIG. 7 a is a cross-section view of an electromagnetic pedicle awlmotivated by a powered chuck in an alternate embodiment of the presentinvention;

FIG. 7 b is an end view of a drill tip of an electromagnetic pedicle awlin an alternate embodiment of the present invention;

FIG. 8 is a pictorial diagram illustrating a preferred electrodearrangement for monitoring a patient's EMG signals while using theelectromagnetic pedicle awl in accordance with the present invention;

FIG. 9 a is a schematic diagram of an RLC circuit suitable for currentpulse generation in the preferred embodiment.

FIG. 9 b is a schematic diagram of an RLC circuit with a shunt diodeacross its capacitor suitable for current pulse generation in thepreferred embodiment.

FIG. 10 a is a magnetic field vector diagram showing the magnetic fieldlines of an electromagnetic pedicle awl in the preferred embodiment ofthe present invention constructed with a core and drill tip of magneticpermeability 1.0.

FIG. 10 b is a set of plots of the relative magnetic flux for a givencurrent time-rate of change in the preferred embodiment of the presentinvention where the core and drill tip is made of material with magneticpermeability 1.0.

FIG. 11 a is a magnetic field vector diagram showing the magnetic fieldlines of an electromagnetic pedicle awl in the preferred embodiment ofthe present invention constructed with a core and drill tip of magneticpermeabilities, 5000 and 300, respectively.

FIG. 11 b is a set of plots of the relative magnetic flux for a givencurrent time rate of change in the preferred embodiment of the presentinvention where the core and drill tip is made of material with magneticpermeabilities 5000 and 300, respectively.

FIG. 12 is a cross-sectional drawing of a side view of theelectromagnetic pedicle awl in an alternate embodiment of the presentinvention where the solenoid coil is integrated into the handle of theelectromagnetic pedicle awl and the core rod of the electromagneticpedicle awl is made of high magnetic permeability material.

FIG. 13 is a cross-sectional drawing of a top-view of the core rod ofthe electromagnetic pedicle awl in an alternate embodiment of thepresent invention.

FIG. 14 a is a magnetic field vector diagram showing the magnetic fieldlines of an electromagnetic pedicle awl in an alternate embodiment ofthe present invention constructed with a core and drill tip of magneticpermeabilities, 5000 and 300, respectively and wherein the solenoid coilis integrated into the handle of the pedicle awl.

FIG. 14 b is a set of plots of the relative magnetic flux for a givencurrent time rate of change in an alternate embodiment of the presentinvention where the core and drill tip is made of material with magneticpermeabilities 5000 and 300, respectively and where the solenoid coil isintegrated into the handle of the electromagnetic pedicle awl.

FIG. 15 a is a cross-sectional drawing of the side view of anelectromagnetic pedicle awl consistent with an alternate embodiment ofthe present invention where the electromagnetic pedicle awl containsmultiple solenoid coils.

FIG. 15 b is a cross-sectional drawing of the top view of anelectromagnetic pedicle awl consistent with an alternate embodiment ofthe present invention where the electromagnetic pedicle awl containsmultiple solenoid coils.

FIG. 16 a is a magnetic field vector diagram showing the magnetic fieldlines of an electromagnetic pedicle awl in an alternate embodiment ofthe present invention constructed with multiple solenoid coils.

FIG. 16 b is a set of plots of the relative magnetic flux for a givencurrent time rate of change in an alternate embodiment of the presentinvention constructed with multiple solenoid coils.

FIG. 17 a is a cross-sectional drawing of an electromagnetic pedicle awlconsisting of an alternate embodiment of the present invention where theelectromagnetic pedicle awl contains a solenoid coil arranged to directa magnetic field.

FIG. 17 b is a cross-sectional drawing of a side view of anelectromagnetic pedicle awl consistent with an alternative embodiment ofthe present invention comprising a solenoid coil arranged to direct amagnetic field.

FIG. 18 is a magnetic field vector diagram showing the magnetic fieldlines of an electromagnetic pedicle awl and an alternate embodiment ofthe present invention constructed with a solenoid winding for directinga magnetic field.

FIG. 19 is a cross-sectional drawing of an electromagnetic pedicle awlincluding a removable core.

FIG. 20 is a cross-sectional drawing of an electromagnetic pedicle awlincluding a removable handle and tip.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The numerous innovative teachings of the present invention will bedescribed with particular reference to the presently preferredembodiments (by way of example, and not by way of limitation).

An electromagnetic pedicle awl is disclosed that utilizes a tightlyfocused time-varying magnetic flux to create a localized electromotiveforce (EMF) near the tip of electromagnetic pedicle awl 200 as shown inFIG. 5 and following. The localized EMF creates corresponding localizededdy currents which excite nearby nerves. The excitation is detected byan electromyographic recording device. In comparison to electrostaticpedicle awl 100 of the prior art, electromagnetic pedicle awl 200excites nerves in close proximity to the tip as opposed to the moregeneral excitation provided by the prior art. Electromagnetic pedicleawl 200 in combination with EMG detector 240 creates a system that ishighly sensitive to the pedicle hole position.

According to Faraday's law of induction for time-varying magneticfields, a voltage or EMF ∈ is induced by a time-varying magnetic flux(pulsed), {right arrow over (B)}(t), and is given by

$ɛ = {{\oint{\overset{->}{E} \cdot \overset{\longrightarrow}{\mathbb{d}l}}} = {{- \frac{\mathbb{d}}{\mathbb{d}t}}{\int_{S}{{\overset{->}{B}(t)} \cdot \overset{\longrightarrow}{\mathbb{d}A}}}}}$

where {right arrow over (E)} is the induced electric field around a loopand the area integral of {right arrow over (B)}·{right arrow over (dA)}is taken over the surface area S of the given loop. The effect of theinduced EMF ∈ in the presence of movable charges or ions is to createeddy currents circulating in a loop perpendicular to the direction ofmagnetic flux {right arrow over (B)}(t). In one case, the magnetic flux,{right arrow over (B)}(t), pulses on and off with pulse duration t_(d)and rise time t_(p).

The classical model of nerve stimulation suggests that E_(th), which isthe threshold electric field necessary for a single pulse to excite anerve to firing, can be expressed as a function of the duration of thepulse d such that:E _(th) =b*(1+g/t _(d)),

where b is the rheobase, the empirically-measured minimum field capableof ever exciting the nerve to fire (e.g., for very long pulsedurations);

g is the chronaxie, the empirically-measured pulse duration that willpermit nerve excitation with a field equal to only twice the rheobase,b, and

t_(d) is the duration of the pulse.

In a simplified illustrative case, a nerve cell may be modeled astangent to a flat disk at right angles to the magnetic field. A simpleapproximation for the tangential electric field E_(φ), tangent to a loopof radius R, wherein the surface area S enclosed by the loop isconsidered flat and orthogonal to the magnetic flux {right arrow over(B)} is given by application of Faraday's law:

$E_{\phi} = {{- \frac{R}{2}}{\frac{\mathbb{d}{B_{\bot}(t)}}{\mathbb{d}t}.}}$

Therefore, nerve stimulation is elicited if:

E_(ϕ) − E_(th) > 0, and${{\frac{\mathbb{d}B_{\bot}}{\mathbb{d}t}} > {\frac{2b}{R}\left( {1 + {g/t_{d}}} \right)}},\text{for}$t:0 < t < t_(d).

Typical values for the human peripheral nerve are:

Rheobase, b: 6-20 V/m for peripheral nerve; and

Chronaxie, g: 200-350 microseconds for peripheral nerve.

Therefore, in the context of the present invention, a pulse duration of30 μs should require an electric field of order 100 V/m to exciteperipheral nerves.

A time varying magnetic flux may be generated by a solenoid coil and maybe calculated off-axis as in the paper, “Some Useful Information for theDesign of Air-core Solenoids” by D. Bruce Montgomery and J. Terrel,November 1961, Air Force contract AF19(604)-7344, incorporated herein byreference. It is sufficient for the purposes of this disclosure to adopta numerical simulation of the magnetic field as accomplished by acomputer program in order to demonstrate that the potential for eddycurrent formation and thus nerve excitation falls off rapidly withdistance from the electromagnetic awl tip.

In order to produce an electromagnetic field around the electromagneticawl, pulsed current source 205 produces a time-varying overdamped orcritically damped set of current pulses, or alternatively a bipolaralternating current. A suitable pulsed current source for theembodiments of the present invention can be functionally modeled as aseries RCL circuit in which a capacitance, C, is charged to an initialvoltage, V₀, and the charge dumped through a coil, L, and resistance, R.

FIG. 9 a shows circuit 380 for a series RCL circuit model of a pulsedcurrent source for a magnetic stimulator. This circuit is improved bylumping the parasitic capacitance, inductance and impedance of thecircuit elements into the values of C, L and R,R=R _(r) +R ₁ +R _(c)L=L ₁ +L _(c)C=C _(c) +C ₁.where R_(r) is the parasitic resistance of the switch, R₁ and L₁ are theequivalent resistance and inductance of a series R-L model of a coilhaving a distributed capacity, C₁.

The current flow for an initial-value RCL problem is arrived at usingelementary circuit analysis (see for example, Smith, R. J., Circuits,Devices and Systems, 4^(th) ed. New York, John Wiley and Sons, 1984).The general solution isi(t)=A·exp(−α·t)·sin(ω·t)where,

i(t) is the current flowing in the circuit at time, t,

A is an amplitude coefficient based upon initial conditions,

α is a damping coefficient such that:

${\alpha = \frac{R}{2L}},$

t is time in seconds,

ω is the angular frequency in radians per second such that:

$\omega^{2} = {\frac{1}{LC} - \frac{R^{2}}{4L^{2}}}$

With initial conditions i(0)=0, i(∞)=0, and L(di/dt)=V₀ at t=0, i(t)becomes

${{\mathbb{i}}(t)} = {\frac{V_{0}}{\omega\; L} \cdot {\exp\left( {{- \alpha} \cdot t} \right)} \cdot {\sin\left( {\omega \cdot t} \right)}}$

The magnitude of the magnetic field produced by the coil is proportionalto the coil current and the induced electric field is proportional tothe time derivative of the current:

$\frac{\mathbb{d}{\mathbb{i}}}{\mathbb{d}t} = {\frac{V_{0}}{L} \cdot {\exp\left( {{- \alpha} \cdot t} \right)} \cdot \left\lbrack {{\cos\left( {\omega \cdot t} \right)} - {\frac{\alpha}{\omega} \cdot {\sin\left( {\omega \cdot t} \right)}}} \right\rbrack}$

According to this result, the maximum di/dt, and hence the maximumelectric field magnitude, occurs at t=0. The maximum current occurs atthe earliest maxima of the current waveform of i(t). Solving the latencyto peak current flow, t_(p), yields:

$t_{p} = {\frac{1}{\omega} \cdot {\tan^{- 1}\left( \frac{\omega}{\alpha} \right)}}$

After the current reaches a peak at time t_(p), it falls until thecurrent direction reverses. Note that zeroes of the current functionoccur when:

${t = {{\frac{n\;\pi}{\omega}:n} = 0}},1,2,\ldots$

The initial zero occurs when the switch is first closed at t=0.Successive zero-crosses occur when n=1, 2, . . . .

By Kirchoff's Law, the voltage across the capacitor must equal the sumof the voltages across the resistor R and inductor L in FIG. 9 a asfollows:

${V_{c}(t)} = {{{{\mathbb{i}}(t)}R} + {L{\frac{\mathbb{d}{\mathbb{i}}}{\mathbb{d}t}.}}}$

If the voltage fraction remaining across the capacitor at any given timeis defined as V_(f)(t)=V_(c)(t)/V₀, then, from substitution of i(t) anddi/dt, the voltage fraction can be determined from the equation:

${V_{f}(t)} = {{\exp\left( {{- \alpha} \cdot t} \right)} \cdot \left\lbrack {{\cos\left( {\omega \cdot t} \right)} + {\left( {\frac{R}{L} - \alpha} \right) \cdot \frac{1}{\omega} \cdot {\sin\left( {\omega \cdot t} \right)}}} \right\rbrack}$

The voltage fraction can be negative which represents a reverse voltageacross the capacitor bank. This voltage can be minimized by using areverse-biased diode shunt D across the capacitor as shown in circuit390 of FIG. 9 b. Since the diode D is in parallel with the capacitor itbegins conducting when V_(c) is negative. Neglecting the diode forwardvoltage drop, the equation for V_(f)(t) can be used to determine whenthe diode conducts by finding the voltage zero preceding a period ofnegative V_(f). The latency to diode forward conduction isapproximately:

$t_{d} = {\frac{1}{\omega} \cdot {\tan^{- 1}\left\lbrack \frac{\omega}{\alpha - {\left( {R_{r} + R_{l}} \right)/L}} \right\rbrack}}$

Notice that the resistance term excludes capacitor internal impedancesince it is shunted by the diode.

With the diode forward biased, the coil current is dissipated by theR_(r) and R₁ such that:

${i_{l}(t)} = {{{{{\mathbb{i}}\left( t_{d} \right)} \cdot {\exp\left( {{- \frac{\left( {R_{r} + R_{l}} \right)}{L}} \cdot t} \right)}}\mspace{14mu}{for}\mspace{14mu} t} > t_{d}}$

An advantage of using a diode-capacitor shunt is that the inducedelectric field is predominately unidirectional. The current pulse has afast positive rise and a slow decay which results in a magnetic fieldhaving a strong initial component in one direction followed by a weakfield in the opposite direction.

In the preferred embodiments of the present invention, the pulsegenerator has approximate component values of inductance L=8.75 μH,capacitance C=7400 μF, resistance R=0.05Ω, a time to peak current, t_(p)of about 400 μs and pulse duration of about 350 μs. Charge voltage V₀ isabout 2 mV and peak current is approximately 0.164 A. A maximum electricfield Ep of about 206 V/m is generated in an eddy current loop ofdimension 4 to 5 mm. Note that Ep>Eth=100 V/m as calculated previouslyfor nerve stimulation to occur

Referring then to FIGS. 5 and 6, an electromagnetic pedicle awl 200 iscomprised of central core 204, removable handle 210, solenoid coil 202encasing central core 204 and pulsed current source 205 for deliveringcurrent pulses to solenoid coil 202 via wire 208. Central core 204 hasdimensions of approximately 10 cm long and approximately 4 mm indiameter. Threaded hole 211 is provided for removable handle 210.Removable handle 210 is provided with a quick disconnect coupler whichmay have the feature of a hexagonal socket for coupling with a hexagonaldriver attached to central core 204. In one embodiment, removable handle210 is provided with a solid rod extension between the grip and thecoupler to extend the reach of the device.

Central core 204 includes drill tip 207 suitable for making pilot holesin bonelike material. In another embodiment the core may be hollow witha removable stylus. The removable stylus aids in cleaning and is used tofit the device with differing stylus tops for various procedures.Solenoid coil 202 includes a wire coil around the non-conductive centralcore 204. Solenoid coil 202 is singly wound approximately one hundredtwenty five (125) turns of 22 AWG insulated copper wire with one end ofthe wire brought through center hole 206 of central core 204 and outnear the handle as wire 208. Wire 208 is connected to pulsed currentsource 205. Wire 209 is locally grounded. In an alternate embodiment,wires 208 and 209 are connected to pulsed current source 205 withdifferential electrical outputs. The wires are connected through arotary coupling 295 which mechanically provides for the rotation of thehandle and the core with respect to the wires.

In one preferred embodiment, central core 204 is constructed of amaterial with low magnetic permeability such as a hardened ceramic. Aparticulate reinforced oxide and non-oxide composite is preferred toincrease mechanical toughness while maintaining low magneticpermeability. Drill tip 207 includes a diamond point or a chisel typeconfiguration for cutting bone material. Drill tip 207 includes a basesubstrate of low magnetic permeability material such as a hardenedceramic

In another preferred embodiment central core 204 contains a materialwith low coercitivity and high magnetic permeability such as siliconsteel or permalloy. Drill tip 207 includes a base substrate of highmagnetic permeability material such as a hardened steel. In this case, aradial slot is provided along the longitudinal axis of the central core.The radial slot eliminates or greatly reduces eddy currents that may begenerated in the high magnetic permeability central core. The eddycurrents are disfavored because they consume energy from the magneticfield being generated by the solenoid and hence reduce its efficiency.

Solenoid coil 202 is covered with a non-conductive polymer sheathing 203so that the surface of pedicle awl 200 is smooth and so that the directcoil drive currents are electrically isolated from the patient. In yetanother embodiment a comb-shaped Faraday shield 226 may be incorporatedcircumferentially between solenoid coil 202 and outer polymer sheathing203 to prevent capacitively coupled electrical stimulation of the nearbynerve tissue. The Faraday shield may also be shaped and positioned todirect the intensity of the electromagnetic field as will be more fullydescribed later. Drill tip 207 outer radius is made to be slightlylarger than the outer surface radius so that pedicle awl 200 may easilypenetrate into the vertebra without obstruction.

The region of nerve excitation 130 is determined by the rapidity ofchange of magnetic flux 225 near the tip. Typically, the pulsed currentsource 205 is made to be adjustable in magnitude and in its pulsecharacteristics such as rise and fall times. Time varying magnetic flux225 in turn induces a localized EMF and corresponding eddy currents inthe vicinity of drill tip 207.

FIG. 10 a shows an example of plot 500 magnetic field lines 230generated from pedicle awl 200 where central core 204 and drill tip 207has a magnetic permeability of 1.1 corresponding to a hardened ceramicmaterial; solenoid coil 202 generates the magnetic field for the samepedicle awl parameters.

EMF produced in loops surrounding drill tip 207 will follow thecharacteristics of magnetic flux strength generated at surroundingpoints, since only a time rate of change of current, di/dt, in solenoidcoil 202 is causing the magnetic flux to change. Relative magnetic fluxfrom one point to another will indicate the relative EMF produced for agiven current rate of change. Relative EMF means EMF at a given spatialpoint normalized to the EMF that produced at the center of the tip. FIG.10 b includes plot 510 of the relative magnetic flux produced at pointsin a spherical arc of radius, r, centered on the tip of the drill tipand at three fixed distances r from the center of drill tip 207 whereinthe relative magnetic flux is the ratio of the magnetic flux at points rto the magnetic flux at the tip, |Br|/|B0|.

The distances, r, are measured in units of solenoid diameter: r=1 unitmeans that the curve 511 was generated for points at a distance of onesolenoid diameter from the tip, curve 512 for points at a distance oftwo solenoid diameters from drill tip 207 and curve 513 for points at adistance of four solenoid diameters from drill tip 207. The points onthe arc are plotted according to their angle where zero degrees is alongthe axis of pedicle awl 200, +90 degrees is perpendicular to the axis ofpedicle awl 200 and to the right of drill tip 207 and −90 degrees isperpendicular to the axis of pedicle awl 200 and straight to the left ofdrill tip 207.

Plot 510 shows that where central core 204 is made of a hardenedceramic, the magnetic field flux is relatively confined, having degradedby 80% at a distance of 4 (four) solenoid diameters from the centerdrill tip 207. Furthermore, plot 510 indicates that the relativemagnetic flux varies by approximately 40% from the forward direction tothe sides, increasing toward the sides. This instrument is more likelyto produce eddy current stimulation of nerves to the side than to theforward direction.

As another example, FIG. 11 a shows plot 520 of magnetic field lines 235generated from pedicle awl 200 where the central core 204 and drill tip207 have a magnetic permeability of 5000 (typical of iron) and drill tip207 has a magnetic permeability of 300 (typical of stainless steel);solenoid coil 202 generates the magnetic field for the same pedicle awlparameters.

FIG. 11 b shows a plot 530 of the relative magnetic flux produced atpoints in spherical arcs of radii, r, centered on the tip of the drilltip and at three fixed distances from drill tip 207 wherein the relativemagnetic flux is the ratio of the magnetic flux at points r to themagnetic flux at the tip, |Br|/|B0|. Curve 531 was generated for pointsat a distance of one solenoid diameter from drill tip 207. Curve 532 wasgenerated for points at a distance of two solenoid diameters from drilltip 207. Curve 533 was generated for points at a distance of foursolenoid diameters from drill tip 207. The points on the arc are plottedaccording to their angle where zero degrees is along the axis of pedicleawl 200, +90 degrees is perpendicular to the axis of pedicle awl 200 andto the right of drill tip 207 and −90 degrees is perpendicular to theaxis of pedicle awl 200 and straight to the left of drill tip 207.

Plot 530 shows that where central core 204 has a high magneticpermeability, the magnetic field flux is well confined, having degradedby almost 90% at a distance of four solenoid diameters from the centerdrill tip 207. Furthermore, plot 530 indicates that the relativemagnetic flux does not vary significantly from the forward direction tothe sides, except for a 20-30% dip near a 45 degree angle from the axisof solenoid coil 202.

In another embodiment electromagnetic probe for probing soft tissue isconceived wherein a pilot hole is not required and thus a drill tip isnot required. FIG. 19 is a cross-sectional drawing of a probe forpassing through soft tissue such as a human muscle. Probe 1900 has solidcore 1902 machined to sharp point 1903. Probe 1900 has solenoid coil1904 around solid core 1902 and non-conductive external cover 1906covering solenoid coil 1904. Solenoid coil 1904 is energized to create atime-varying magnetic field in the vicinity of point 1903.

In yet another embodiment, the electromagnetic awl utilizes an air coreinstead of a solid core for transmitting magnetic flux through asolenoid coil. FIG. 20 is a cross sectional drawing of electromagneticawl 1950 comprising a non-conductive cylindrical tube 1952 with threads1957 tapped at a lower end and threads 1961 tapped at an upper end.Electromagnetic awl 1950 further comprises solenoid coil 1954 aroundcylindrical tube 1952 and non-conductive external cover 1956 coveringsolenoid coil 1954. Drill tip 1958 with threads 1959 is fastened intothe lower end of electromagnetic awl 1950 such that threads 1959 aremated with threads 1957. Handle 1960 with threads 1962 is fastened ontothe upper end of electromagnetic awl 1950 such that threads 1962 aremated with threads 1961. Alternatively, a vertically oriented handlewith long axis along the same axis as electromagnetic awl 1950 may beused in place of the horizontally oriented handle as shown. Other waysof fastening the handle and the drill tip to the cylindrical tube may beused; such as a suitable epoxy adhesive.

An alternate embodiment of the present invention is shown in FIG. 12 aselectromagnetic pedicle awl 600. Pedicle awl 600 is constructed of corerod 605 to which is attached a handle 620 and drill tip 608. Handle 620may be connected to core rod 605 by a quick disconnect or by threadedscrew or by other suitable means. Drill tip 608 may be machined as partof core rod 605 or it may be a removable tip capable of replacement.Solenoid coil 610 is mechanically integrated into handle 620 using epoxyor other suitable means and surrounds core rod 605 near its handle end.Referring to FIGS. 12 and 13, core rod 605 and drill tip 608 includeradial slot 609 along their longitudinal axes in order to reduce eddycurrents within core rod 605.

Solenoid coil 610 in this preferred embodiment is a multi-layer coilhaving approximately 200 wraps. Solenoid coil 610 is connected via wires615 to a pulsed current source. When pulses of current are sent throughsolenoid coil 610 a magnetic flux is generated internal to its coil andcore rod 605. Core rod 605 is preferably made with a high permeabilitymetal such as iron or steel. The core rod serves as a magnetic conduitfor magnetic flux to propagate from solenoid coil 610 down and throughdrill tip 608. Drill tip 608 is also made of a material with significantmagnetic permeability such as steel.

FIG. 14 a shows a plot 540 of magnetic field lines 630 generated frompedicle awl 600 where the core rod 605 has a magnetic permeability of5000 and drill tip 608 has a magnetic permeability of 5000; solenoidcoil 610 generates the magnetic field.

EMF produced in loops surrounding drill tip 608 will follow thecharacteristics of magnetic flux strength generated at surroundingpoints, since only a time rate of change of current, di/dt, in solenoidcoil 610 is causing the magnetic flux strength to change. Relativemagnetic flux field from one point to another will indicate the relativeEMF produced for a given current rate of change.

In FIG. 14 b is plot 550 of the relative magnetic flux produced atpoints in spherical arcs of radii, r, centered on drill tip 608 and atthree fixed distances r from drill tip 608 wherein the relative magneticflux is the ratio of the magnetic flux at points r to the magnetic fluxat the tip, |Br|/|B0|.

The curve 551 was generated for points at a distance of one solenoiddiameter from drill tip 608. Curve 552 was generated for points at adistance of two solenoid diameters from drill tip 608 and curve 553 wasgenerated for points at a distance of four solenoid diameters from drilltip 608. The points on the arc are plotted according to their anglewhere zero degrees is along the axis of pedicle awl 600, +90 degrees isperpendicular to the axis of pedicle awl 600 and to the right of drilltip 608 and −90 degrees is perpendicular to the axis of pedicle awl 600and straight to the left of drill tip 608.

Plot 540 of FIGS. 14 a and 550 of FIG. 14 b show that where core rod 605is made of a high magnetic permeability material, the magnetic fieldflux is relatively confined, having degraded by about 75% at a distanceof four solenoid diameters from the center drill tip 608. Furthermore,plot 550 indicates that the relative magnetic flux varies byapproximately 30% to 40% from the forward direction to the sides,increasing toward the forward direction. This instrument is more likelyto produce eddy current stimulation of nerves to the forward directionthan to the sides.

Another alternate embodiment of the present invention is shown in thecross-section drawings of FIGS. 15 a and 15 b as pedicle awl 700.Pedicle awl 700 is comprised of four solenoid coils encased within acore assembly 705 to which is attached a handle 720 and drill tip 708.Wire leads 715 are attached to each solenoid coil sufficient toindependently drive a current through each solenoid coil. Typicaldimensions for pedicle awl 700 is an outer surface diameter of about 4.5mm, solenoid coil diameter 723 of 1.6 mm, and length from handle 720attachment point of core assembly 705 to drill tip 708 attachment pointof core assembly 705, approximately 7 cm and the solenoids use 32 AWGmagnet wire. In the currently described embodiment, the central cores ofeach solenoid, the core assembly 705 and drill tip 708 are made of a lowmagnetic permeability material. Alternate embodiments include highmagnetic permeability materials used for the core assembly and drilltip.

Wire leads 715 are attached to a switching pulsed current source 706.The switching pulsed current source 706 is connected to a digitalcontroller 709 and display 707. The digital controller energizes eachsolenoid coil in a rotating sequence over time thereby producing arotating directional field. The rotating sequence in the preferredembodiment ranges from about 1 cycle per second to about 25 cycles persecond. The given arrangement of solenoid coils for pedicle awl 700allows for directional probing for nearby nerves. As each coil isenergized, the side nearest the energized coil will generate a largermagnetic flux strength which in turn will interact most strongly withnerves in its vicinity. The position of the active coil is shownrelative to the handle by a graphic display generated by the controllerand displayed on display 707. The rotating directional field is usefulin applications where the location of the nerve with respect to the axisof the awl is important. The rotating directional field is also usefulin robotic applications as a feed back mechanism for correcting theentry angle of the awl during surgery.

In an alternate embodiment, more than one coil may be energized at atime to produce different geometries of magnetic flux strength. Forexample, in one embodiment, a four-coil geometry is adopted, havingcoils 180 degrees apart with respect to the axis of the pedicle awl.Each coil can be energized with current of differing polarization,producing opposite fields in the respective coils. The opposite fieldsin adjacent coils produce a distinctively biased magnetic field. In use,as two opposing energized coils are de-energized the opposing coilsimmediately clockwise are energized according to the polarity of itsimmediate neighbor in a rotating fashion. The process effectivelycreates rotating magnetic flux “beam” which sweeps about the pedicle awlwith respect to its longitudinal axis at rotation speed set by thedigital controller.

In yet another embodiment of the present invention, a useful display iscreated by digitally combining the radial position of the energized coilwith digital EMG signals within the digital controller. A graphicalcorrelation of the EMG signal strength with a magnetic field positioncreates a graphical map of the nerve cells. In the case of vertebralpedicles, the graphical map represents a map of the pedicle wall. In thecase of the psoas muscle, the graphical map represents the location ofperipheral nerves to be avoided.

FIG. 16 a is plot 558 of magnetic field lines 730 generated from pedicleawl 700 for a given situation where the leftmost solenoid coil 710 isenergized. Solenoid coil 710 generates the magnetic field as shown. Themagnetic field lines are clearly skewed to the left, offset by theposition of solenoid coil 710 with respect to the center axis of coreassembly 705. The plots of FIGS. 16 a and 16 b were generated for thepedicle awl parameters M_(rod)=5000, M_(solenoid core)=5000.

EMF produced in loops surrounding drill tip 708 will follow thecharacteristics of magnetic flux strength generated at surroundingpoints, since only a time rate of change of current, di/dt, in solenoidcoil 710, solenoid coil 711, solenoid coil 712 or solenoid coil 713 iscausing the magnetic flux to change. In FIG. 16 b is plot 560 of therelative magnetic flux produced at points on spherical arcs of radii, r,centered on drill tip 708 and at three fixed distances r from drill tip708 wherein the relative magnetic flux is the ratio of the magnetic fluxat points r to the magnetic flux at the tip, |Br|/|B0|.

Curve 561 was generated for points at a distance of one solenoiddiameter from tip 708. Curve 562 was generated for points at a distanceof two solenoid diameters from drill tip 708. Curve 563 was generatedfor points at a distance of four solenoid diameters from drill tip 708.Solenoid diameters for solenoid coil 710, solenoid coil 711, solenoidcoil 712 and solenoid coil 713 are shown to be identical, but may bedifferent in other embodiments of the present invention.

Plot 560 shows that according to the alternative embodiment of thepresent invention, wherein core assembly 705 is made of a high magneticpermeability material, the magnetic field flux is relatively confined onone side but not on the other, having degraded by about 75% at adistance of four solenoid diameters from drill tip 708 to the rightside. Plot 560 indicates that the relative magnetic flux varies byapproximately 50% to 60% from the left side direction to the right sidedirection, increasing toward the left side direction. This instrument,with solenoid coil 710 energized, is more likely to produce eddy currentstimulation of nerves to the left side direction than to the front or tothe right side. Not shown are similar curves when the other threesolenoids are energized which may be inferred by symmetry.

In FIGS. 7 a and 7 b, a cross-sectional view of an alternate embodimentof the present invention is shown. Electromagnetic pedicle awlsub-assembly 300 is comprised of awl 307, central core bushing 304 forholding awl 307, including shaft 310, solenoid coil 302 surrounding corebushing 304 and a multi-jawed chuck 387 for gripping the awl androtating it with respect to core bushing 304. Shaft 310 extends out ofcore bushing 304 and is attached by the chuck. Shaft 310 includeslongitudinal slot 306 to reduce eddy current. Solenoid coil 302 ismechanically fixed to core bushing 304. Awl 307 rotates with respect tocore bushing 304. A forward thrust bearing 389 is rigidly attached toshaft 310. Forward thrust bearing 389 is directly adjacent to and insliding contact with forward thrust surface 385 on core bushing 304. Arear thrust bearing 320 is provided on shaft 310 adjacent drill tip 308and is rigidly attached to shaft 310. Rear thrust bearing 320 isadjacent to and in sliding contact with rear thrust surface 321 on drilltip 308. In use, the forward thrust surface and forward thrust bearingin cooperation provide sufficient force to maintain the axial positionof the core bushing with the shaft during forward drilling operations.The reverse thrust bearing and reverse thrust surface in cooperationmaintain the axial position of the shaft and the core bushing as the awlis withdrawn from the pedicle.

The two ends of the coil are connected to connector block 350 which ispositioned on the exterior of the solenoid coil 302 and is stationaryduring the rotation of the drill. The connector block serves as anelectrical connection point for connector 353 which is connected towires 355 that provide drive current for the solenoid coil. Drill tip308 includes the features of a conical surface with shoulder 309sufficient to move material away from the outer surface of solenoid coil302. The chuck in turn is attached to a handle such as that described inrelation to FIG. 6 or attached to a robot end effector. In yet anotherembodiment, a motor or impact wrench may be attached to the chuck sothat controlled rotation and axial impacts may be delivered to the awl.Solenoid coil 302 is connected to a time-varying current source whichmay produce overdamped, critically damped current pulses or a bipolaralternating current.

Awl 307 is preferably made of a non-conductive material and may be amachined composite ceramic. Core bushing 304, preferably made with anon-ferromagnetic material, has a slot 299 along its axis to avoid thebuildup of eddy currents. Alternatively core bushing 304 may be made ofa machined ceramic. Referring to FIGS. 7 a and 7 b, drill tip 308 issimilar to a twist drill bit and is preferably made with anon-ferromagnetic material with straight slot 298 along its axis toavoid eddy currents. FIG. 7 b shows an end view of drill head 308. Inthis view, an alternate configuration is shown having a series ofhelical slots 326 that function to eliminate eddy currents in drill tip308 and move debris out of the way during use.

FIGS. 17 a and 17 b show yet another embodiment of the presentinvention. In this embodiment, an awl 1800 is provided with a centralnonconductive core 1850. Around the central nonconductive core, a coil1854 is placed including return windings 1856 and complete windings1860. Return windings 1856 double back around 180° of the circumferenceof the non-conductive core for every complete rotation of the completewindings 1860. The return windings operate to cancel the magnetic fieldgenerated by the complete windings 1860 immediately adjacent.

The awl is covered with a non-conductive external cover 1858 and fittedwith drill tip 1852.

Referring to FIG. 18, a plot of the magnetic field generated by thealternative embodiment of FIGS. 17 a and 17 b is shown. It can be seenthat the magnetic field density is larger on the side of the awlopposite return windings 1856. Return windings 1856 in conjunction withthe complete windings 1860 provide for directional projection of themagnetic field.

The embodiments of the present invention are typically applied to thelumbar area of the spine in practice, but may be also applied to thecervical and thoracic spine.

Referring again to FIGS. 5 and 6, the method of use of theelectromagnetic pedicle awl is described. In use, an EMG detector 240 issuitably attached to a patient (not shown) with electrodes placed in themuscles of the lower extremity known to respond to specific nerveslocated outside pedicle wall 222.

FIG. 8 is a pictorial diagram of the muscles 400 of the lower extremity.The nerves for the muscles 400 of the lower extremity are known to belocated in the lumbar 2 and sacral 5 regions of spinal column 1. It iseffective to place EMG electrodes on muscles 400 and monitor them forelectrical impulses brought about by excitation of nerves in thevertebral foramen during pedicle hole placement. In the preferredembodiment of the present invention, electrodes are placed on the vastuslateralis muscle 405, the medial gastrocnemius muscle 406 and tibialisanterior muscle 407. As those skilled in the art can appreciate, manyother embodiments of the present invention are realized by utilizing anumber of combinations of electrode placement on the muscles 400 of thelower extremity and other places on the human body. As such, the presentinvention is not limited by the specific location of EMG electrodes: theauthor simply desires to indicate a preferred placement of EMGelectrodes.

Referring to FIGS. 5 and 6, electromagnetic pedicle awl 200 is initiallypositioned by fluoroscopic imaging onto pedicle 220 of vertebra 215 andthen rotated using removable handle 210 or lightly impacted with ahammer (not shown) to create a pilot hole through pedicle 220. Aselectromagnetic pedicle awl 200 is passed through the pedicle 220, EMGdetector 240 is monitored for signals that indicate that nerveexcitations are resulting from the induced current due to the proximityof the awl tip. In embodiments which include a digital controller forcontrolling the sweep rate of the electromagnetic field, the sweep rateis set to a predetermined rate. In one preferred embodiment, thepredetermined rate varies between one cycle per second and ten cyclesper second. In embodiments where a graphical display is provided, thegraphical display is monitored for the correlation between the placementof the pedicle awl and response generated by the EMG detector. Inresponse to the EMG detection signals, pedicle awl 200 is redirected tocorrect placement error.

When used with other surgical techniques, such as in the XLIF procedure,a similar procedure is employed. An EMG detector is suitably attached tothe patient with electrodes placed in muscles known to respond tospecific nerves within the psoas muscle. The projected entry site of thepsoas muscle is exposed and an electromagnetic awl is positioned byintraoperative imaging. A pilot hole is created at the surface of psoasmuscle. The EMG detector is monitored as the electromagnetic awl passesthrough psoas muscle for nerve excitations that result from the flow ofionic current in the nerve due to the localized EMF. If nerveexcitations occur, placement corrections can be made to avoid nervedamage.

It should be emphasized that the above-described systems and methods ofthe present invention, particularly, any exemplary embodiments, aremerely possible examples of implementations and are merely set forth forproviding a clear understanding of the principles of the invention. Thedescriptions in this specification are for purposes of illustration onlyand are not to be construed in a limiting sense. Many variations will beapparent to persons skilled in the art upon reference to the descriptionand may be made to the above-described embodiments of the inventionwithout departing substantially from the spirit and principles of theinvention. All such modifications and variations are intended to beincluded herein within the scope of this disclosure and the presentinvention and protected by the following claims.

1. A surgical awl for use in surgical procedures comprising: a solidcore having a first end and a second end; a tapered awl tip attached tothe first end of the solid core; a handle attached to the second end ofthe solid core; an electrically conductive coil around the solid core;an electric current source, connected to the electrically conductivecoil, capable of producing a time varying electrical signal; the timevarying electrical signal producing a time varying magnetic field in theconductive coil adjacent the awl tip; the time varying magnetic field ofan intensity to produce an electronically measurable excitation a set ofliving nerve cells; and, a detector for monitoring a set of muscles forthe electronically measurable excitation.
 2. The surgical awl of claim 1wherein the solid core is non-conductive.
 3. The surgical awl of claim 1wherein the solid core is ceramic.
 4. The surgical awl of claim 1wherein the awl tip is fixed with respect to the conductive coil.
 5. Thesurgical awl of claim 1 further comprised of a quick disconnect forattaching the handle to the solid core.
 6. The surgical awl of claim 1wherein the magnitude of the electric signal is adjustable.
 7. Thesurgical awl of claim 1 wherein the electric signal automatically varieswith time.
 8. The surgical awl of claim 7 wherein the electric signal isone of the group of bipolar alternating, critically damped, oroverdamped.
 9. The surgical awl of claim 7 wherein the electric signalis pulsed.
 10. The surgical awl of claim 1 wherein the detector is anelectromyograph connected to the set of muscles.
 11. The surgical awl ofclaim 10 wherein the set of muscles includes one of the group of vastuslateralis muscle, medial gastrocnemius muscle, or tibialis anteriormuscle.
 12. The surgical awl of claim 1 wherein the time varyingmagnetic field induces a time varying electric field that exceeds arheobase value associated with the set of living nerve cells.
 13. Thesurgical awl of claim 1 wherein the solid core is rotatably connected tothe conductive coil.
 14. The surgical awl of claim 1 wherein the awl tipincludes a slot for reducing eddy currents.
 15. The surgical awl ofclaim 1 wherein the conductive coil is covered with a shield of highdielectric strength.
 16. The surgical awl of claim 1 wherein theconductive coil includes a set of full windings and a set of returnwindings adjacent the tapered awl tip.
 17. The surgical awl of claim 1wherein the surgical procedure is creating a hole in a pedicle.
 18. Thesurgical awl of claim 1 wherein the surgical procedure is creating apassageway through a psoas muscle.
 19. The surgical awl of claim 1wherein the time varying magnetic field obeys the equation${\frac{\mathbb{d}B_{\bot}}{\mathbb{d}t}} > {\frac{2b}{R}\left( {1 + {g/t_{d}}} \right)}$where: b=the rheobase in volts/meter; g=the chronaxie in seconds;t_(d)=the pulse duration in seconds; and R=loop radius.
 20. A surgicalawl tool for creating a pilot hole in bony material proximate to nervecells, the tool capable of producing magnetic fields comprised of: asolid core bushing; a tapered drill, having a drill tip and acylindrical shaft, the cylindrical shaft resident in the solid corebushing so that a portion of the cylindrical shaft protrudes axiallyfrom the solid core bushing; the cylindrical shaft having a reversethrust bearing attached near the tapered drill and set against the solidcore bushing; the cylindrical shaft having a forward thrust bearingattached to the cylindrical shaft and set against the end of the solidcore bushing so as not to allow lateral movement of the tapered drillwith respect to the solid core bushing; a coil assembly for producing amagnetic field further comprised of a conductive coil of wire around thesolid core bushing; an electric current source attached to theconductive coil of wire so that a time varying magnetic field is capableof being generated in the vicinity of the drill tip of an intensitysufficient to produce an electronically measurable excitation of thenerve cells; and, a mechanism for detecting the excitation of the nervecells by the time varying magnetic field as the drill tip penetrates thebony material.
 21. The surgical awl of claim 20 wherein the drill tip iscapable of rotation with respect to the solenoid assembly.
 22. Thesurgical awl of claim 20 wherein the cylindrical shaft is rigidlyattached to a gripping handle.
 23. The surgical awl of claim 20 whereinthe shaft is attached to a robot end effector.
 24. The surgical awl ofclaim 20 wherein the solid core bushing is non-conductive.
 25. Thesurgical awl of claim 20 wherein the electric current source produces asignal that varies with time.
 26. The surgical awl of claim 25 whereinthe signal is one of the group of critically damped, overdamped orbipolar alternating pulsed current.
 27. The surgical awl of claim 20wherein the mechanism is an electromyograph.
 28. A surgical awl for usein surgical procedures comprising: a hollow cylindrical core having afirst end and a second end; a tapered awl tip attached to the first endof the hollow cylindrical core; a handle attached to the second end ofthe hollow cylindrical core; an electrically conductive coil around thehollow cylindrical core; an electric current source, connected to theelectrically conductive coil, producing a time varying electricalsignal; the time varying electrical signal producing a time varyingmagnetic field in the conductive coil adjacent the awl tip; the timevarying magnetic field of an intensity to produce an electricallymeasurable excitation in a set of living nerve cells; and, a detectorfor monitoring a set of muscles for the electrically measurableexcitation.
 29. The surgical awl of claim 28 wherein the time varyingmagnetic field obeys the equation${\frac{\mathbb{d}B_{\bot}}{\mathbb{d}t}} > {\frac{2b}{R}\left( {1 + {g/t_{d}}} \right)}$where: b=the rheobase in volts/meter; g=the chronaxie in seconds;t_(d)=the pulse duration in seconds; and R=loop radius.
 30. The surgicalawl of claim 28 wherein the hollow cylindrical core is non-conductive.31. The surgical awl of claim 28 wherein the hollow cylindrical core isceramic.
 32. The surgical awl of claim 28 wherein the tapered awl tip isfixed with respect to the electrically conductive coil.
 33. The surgicalawl of claim 28 further comprised of a quick disconnect for attachingthe handle to the hollow cylindrical core.
 34. The surgical awl of claim28 wherein the magnitude of the electric signal is adjustable.
 35. Thesurgical awl of claim 28 wherein the electric signal automaticallyvaries with time.
 36. The surgical awl of claim 35 wherein the electricsignal is one of the group of bipolar alternating, critically damped andoverdamped.
 37. The surgical awl of claim 35 wherein the electric signalis pulsed.
 38. The surgical awl of claim 28 wherein the detector is anelectromyograph connected to a set of muscles.
 39. The surgical awl ofclaim 38 wherein the set of muscles includes one of the group of vastuslateralis muscle, medial gastrocnemius muscle and tibialis anteriormuscle.
 40. The surgical awl of claim 28 wherein the hollow cylindricalcore is rotatably connected to the conductive coil.
 41. The surgical awlof claim 28 wherein the tapered awl tip includes a slot for reducing aneddy current.
 42. The surgical awl of claim 28 wherein the electricallyconductive coil is covered with a shield of high dielectric strength.43. The surgical awl of claim 28 wherein the electrically conductivecoil includes a set of full windings and a set of return windingsadjacent the tapered awl tip.
 44. The surgical awl of claim 28 whereinthe surgical procedure is creating a hole in a pedicle.
 45. The surgicalawl of claim 28 wherein the surgical procedure is creating a passagewaythrough a psoas muscle.
 46. The surgical awl of claim 28 furthercomprising: an outer casing adjacent the electrically conductive coil;and, a faraday shield integrated into the outer casing.